Dynamic x-ray imaging is used extensively in medicine to provide image guidance to cardiologists, gastroenterologists, pulmonologists, interventional radiologists, orthopedic surgeons and radiation oncologists. Often, a tool or target is visualized during such dynamic x-ray imaging. For example, fluoroscopy is used during cardiac catheterization to visualize the guidewire as it is inserted, and cone-beam computed tomography (CBCT) is used during orthopedic spine procedures to enhance localized placement of screws.
Dynamic x-ray imaging is also used in radiation therapy to maximize the radiation dose distributed to target volumes (e.g., cancerous growths) in a patient's anatomy while minimizing the dose delivered to surrounding tissues. This practice is referred to as image guided radiation therapy (IGRT), and it allows for increased accuracy in distributing targeted radiation doses to complex target volumes. In particular, IGRT provides a feedback loop that enables adjustment of a patient's treatment based on an assessment of the patient's three-dimensional anatomy just prior to radiation beam delivery. This is especially helpful, as a patient's anatomy continually changes due to physiological changes and treatment responses.
In IGRT based radiation therapy, pre-planning computed tomography (CT) data is used to plan dose distributions while markers are used to line up the patient with the pre-planned dose distribution on treatment day. However, complications arise when the target volume in the patient's anatomy is in motion during the treatment. In particular, movement of the target volume during delivery of the radiation beam therapy may result in an underdosing to the target volume (due to its movement away from the radiation beam), and overdosing of the surrounding normal tissue (due to its movement into the radiation beam). Such target volume motion is often experienced in the treatment of lung, liver, and pancreatic cancers where the relevant anatomy moves due to respiratory motion from a patient's breathing. Target volume motion may also be experienced due to other involuntary muscle movements, such as when the target volume is positioned proximate to cardiac or gastrointestinal muscles. Target volume motion can be clinically significant, on the order of 2-3 cm, depending on target volume site and other patient specific circumstances. See S B Jiang, Technical Aspects of Image-Guided Respiration-Gated Radiation Therapy; Medical Dosimetry; 31(2):141-151, 141; 2006. As such, target volume movement can cause large differences between the planned and actual dose distributions, including potential damage to essential surrounding tissues and degraded efficacy of the treatment.
Target volume motion errors may be accounted for by setting dose margins during pre-planning. However, without imaging, a treating physician cannot know whether a target volume remains within the pre-planned boundaries. Thus, many physicians will use a gating technique during therapy. One gating technique is the use of a gating window, wherein the delivery of a radiation dose is timed to correspond with the target volume motion such that the radiation dose is delivered only during periods when the target volume is judged to be in a predetermined path of the radiation beam. Another gating technique is beam-tracking, wherein the radiation beam is made to track the movement of the target volume such that the predetermined path of the radiation beam remains aligned with the moving target volume. This is similar to other image guided interventions, where the practitioner may move the tool to intervene at the correct time and position for the imaged object.
Gating techniques account for movement of a target volume by utilizing external or internal motion surrogates to approximate the target volume motion. In particular, an imaging-conducive structure proximate to a target volume, and which is expected to move in a similar manner to the target volume, is selected to serve as a motion surrogate. The movement of the motion surrogate is then monitored and used to derive a projected motion for the corresponding target volume. Gating techniques using external motion surrogates may be practiced with a system such as the Real-time Position Management (RPM) system, available from Varian Medical Systems (Palo Alto, Calif.). However, studies have shown that external motion surrogates do not always correlate well with actual target volume motion, and are dependent on the placement of the external surrogate. Thus, internal motion surrogates, implanted near the target volume, are preferred when possible. Gating techniques using internal motion surrogates may be practiced with a system such as the Real-time Tracking Radiation Therapy (RTRT) system, available from Mitsubishi Electronics Co., Ltd. (Tokyo, Japan). Suitable structures for use as an internal motion surrogate may include the host organ to the target volume; nearby anatomical structures (e.g., the diaphragm or chest wall for a target volume in the lung); radiopaque markers implanted at the target volume; and radioactive tracers introduced to the target volume. Fiducial markers, which, unlike soft tissue target volumes, are radiopaque and can be seen in x-ray images, are perhaps the most widely used internal motion surrogate.
In a typical clinical setting, one or more fiducial markers are implanted on or near the target volume prior to acquisition of the pre-planning CT images. The markers are then used as a motion surrogate for pre-planning a dose distribution that accounts for a target volume movement. On the day of treatment, immediately prior to dose delivery, CBCT images of the patient are taken from different gantry angles, and a reconstructed three-dimensional (3D) volume is generated from the CBCT images to model the patient's internal anatomy. The reconstructed 3D volume is then used in combination with fluoroscopic images to manually align the fiducial marker locations to those in the pre-planning CT, and to position the patient on the treatment couch. The motion of the fiducial markers in the captured images, corresponding to the target volume motion, is used to specify a gating technique for the treatment. For example, in a gating window technique, the motion of the fiducial marker may be used to manually set the gating window (specifying when the radiation beam is on), such that the radiation dose is delivered only when the target volume is within a specified region corresponding to the fixed path of the radiation beam.
Tracking and localization of fiducial markers in the CBCT images is necessary for discerning the motion of the fiducial markers that are used as motion surrogates in specifying a gating technique and pre-planning delivery of a prescribed radiation dose to a patient. Currently, all known methods for monitoring and localizing fiducial markers in CBCT images use a template matching algorithm, or a derivative thereof. Template matching algorithms use CBCT images to identify a fiducial marker based on a priori information relative to the marker shape and size. In particular, template matching algorithms require a pre-loaded template for matching with the CBCT images to identify a fiducial marker having a corresponding shape and size.
A serious limitation of the conventional template matching algorithms, however, is that they often fail to correctly identify fiducial markers in CBCT images when the images have low contrast (e.g., low signal-to-noise ratio), or when the markers are relatively small or of an irregular shape. This limitation is significant as physicians are moving away from gold spheres and cylinders (which normally present high contrast and a symmetrical shape profile) and more towards coiled fiducial markers. In particular, some consider the coiled fiducial markers easier to implant due to the smaller size, as well as less likely to migrate due to their irregular shape. However, these coiled markers present irregular shapes and a lower contrast as compared to the gold spheres and cylinders. Also, when used with a template matching algorithm, these coiled markers require a treatment operator to manually create a reference template corresponding to the irregular shape of the coiled marker, as the coiled marker will take on a unique shape upon insertion. As such, the performance of a conventional template matching algorithm using a coiled marker greatly depends on the quality of the template, and is subject to the introduction of considerable user errors in generating the template.
Accordingly, the currently available methods and systems for marker tracking, which require a priori information such as a pre-loaded template, are expected to yield evermore inconsistent results as the use of irregularly shaped and sized markers becomes more prevalent. Therefore, there remains a need in the art for a marker tracking method and system that is capable of providing consistently reliable results regardless of marker type, shape, size, and orientation.